Champagne to Mosel

You have lots of options. Try Verdun for WWI related sites. There is an amazing museum in Bastogne, Belgium related to the Battle of the Bulge. You could visit Luxembourg City, quite lovely. Another idea is Trier, Germany. There's a Roman gate, cathedral and medieval town center.

Assume you are renting the car in France and returning it to France to avoid high international drop fees.

From Reims to Trier is a pretty fast drive if you take the A4 (France) A6 (Germany). But it's only marginally attractive and the tolls will run around 60 euro. If you use the Waze app you can ask it to avoid toll roads. Then it will probably send you through some small towns like Grandpre or Brieulles-sur-Meuse. That's a much nicer drive, although slower, and on secondary roads. But I prefer those.

If you do that you will be cutting right across the (large) battlefield where Americans fought in WWI (and again just 30 years later.) There are all kinds of monuments, big and small, throughout that area. If that's attractive to you consider staying in Cheppy, or Varennes-en-Argonne. (Varennes is where Louis XVI and Marie Antoinette were captured trying to escape the Revolution.) These are smaller towns, very pretty, and worth a visit.

Check out the American Battlefield Monuments Commission website for some really interesting places to visit along the way. There are dozens, but the ABMC are the best. https://www.abmc.gov/cemeteries-memorials/